In the art of upward acting doors, it is conventional practice to provide a counterbalance or door lifting assist system comprising an elongated shaft mounted generally above and adjacent to the door, the shaft including spaced apart cable drums for counterbalance cables which are connected to the opposite lower edges of the door. Counterbalance forces are typically provided by one or more torsion coil springs sleeved over the shaft between the cable drums and connected at one end to an anchor bracket and at an opposite end to the counterbalance shaft or directly to one of the cable drums. Conventional counterbalance systems also require manual winding of the torsion springs, a longstanding, nettlesome and somewhat hazardous operation. Spring replacement is also difficult and normally requires disassembly of the counterbalance system or at least major portions thereof.
Certain improvements in torsion spring type counterbalance systems for upward acting doors have been provided, such as disclosed and claimed in U.S. Pat. No. 6,134,835, assigned to the assignee of the present invention. However, additional improvements have been sought in counterbalance systems for upward acting doors and it is to these ends that the present invention has been developed.